1. Field of the Invention
The present invention relates generally to the field of control systems. More specifically, the present invention discloses a control system for counter-oscillating masses, such as found in a light detection and ranging (LIDAR) spacecraft, to provide overall controller and system structural modal stability.
2. Statement of the Problem
Spacecraft have been used for a number of years for remote sensing of earth environmental concerns. Some of these spacecraft have employed small oscillating mirrors to optically scan earth features. An application of this type, using a large mirror, has been proposed for a LIDAR payload. A large oscillating inertia on a spacecraft, even when a counterbalance is used, can couple through the scanning controller with flexible spacecraft structure to cause structural modal instability of the overall system. The requirement to stabilize the overall spacecraft structural system including the controller is the primary motivation behind this invention.
Active remote sensing techniques using lasers such as LIDAR and laser altimetry, have proven useful for many years. While such methods from ground-based and airborne platforms are fairly mature, active remote sensing from space-based platforms is still very much in its infancy. However, the global reach of space-based methods make them highly attractive for a wide range of active remote sensing activities such as detection of atmospheric pollution, global wind measurement, topological, chlorophyll and mineral mapping, and global climate monitoring. An important requirement for space-based LIDAR-type systems is the ability to accurately scan a laser spot and collect the scattered return signal. In general, this translates to the ability to precisely scan a large aperture mirror system. Throughout the remainder of the present application, the word "system" should be construed as including, but not being limited to a LIDAR or similar space-based system.
FIG. 1 is a simplified schematic diagram of the scanning device, including a mirror 12, a counterbalance 16, and an actuator 14 for rotating the mirror 12 and counterbalance 16 about a common axis. The mirror 12 oscillates in a sinusoidal scanning motion with an amplitude of plus and minus a certain number of desired degrees. The oscillation is about a center position which could be nadir or any other commanded center position. The counterbalance 16 is driven oppositely to the mirror 12 to produce, in the ideal case, zero net angular momentum. A closed-loop controller commands and stabilizes the oscillating motion of the mirror 12 and counterbalance 16.
Research into controller designs uncovered an unexpected result in which the actuator 14, closed-loop controller, and flexible spacecraft structure 18 couple to create an unstable condition. The instability was unexpected because the actuator is designed, with the mirror 12 and counterbalance 16 oscillating in opposite directions, to produce very little net force and torque on the mounting structure. The actuator 14 is attached to a flexible mounting structure 18 that is in turn attached to a spacecraft bus 19 that hosts the system payload. Other possible flexible elements of this system are the bus 19 itself and appendages such as solar arrays and booms supporting antenna and other instrument packages. All such flexible elements can contribute to the observed instability. Straight forward application of a conventional PID (proportional, integral, differential) controller was found not to be sufficient to stabilize this phenomenon. Therefore, a need exists for a control system capable of preventing such instability in this type of system.
3. Solution to the Problem
The present invention provides a control system for a highly precise and efficient mechanical scanning device suitable for use in this type of system. In particular, the present invention employs feedback control with a high bandwidth channel for the mirror and a low-bandwidth channel for the counterbalance. The counterbalance control channel includes torque cross-feed from the mirror control channel, and a notch filter to remove the commanded oscillation frequency.